The Three States of Matter (Edexcel IGCSE Chemistry)

Spec point — Understand the three states of matter in terms of the arrangement, movement & energy of the particles

States of matter

• The three states of matter are solids, liquids and gases

• A substance can usually exist in all three states, dependent on temperature (and pressure)

• State changes occur at the melting point (solid to liquid, liquid to solid) and at the boiling point (liquid to gas and gas to liquid)

  • Melting and freezing occur at the melting point
  • Boiling and condensing take place at the boiling point

• Individual atoms themselves do not share the same properties as bulk matter

• The three states of matter can be represented by a simple model

  • In this model, the particles are represented by small solid spheres

Summary of the properties of the three states of matter

Interconversion Between the States of Matter

Spec point — Understand the interconversions between the three states of matter in terms of:

• the names of the interconversions
• how they are achieved
• the changes in arrangement, movement & energy of the particles

Changing states of matter

• The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles

• The stronger the forces between the particles, the more energy that is needed to overcome them

• Therefore, the stronger the forces between the particles the higher the melting point and boiling point of the substance

• Changing states is a physical change

  • The particles themselves remain the same, it is just the forces between the particles which change

Melting

• Melting is when a solid changes into a liquid
• Heat / thermal energy absorbed by the particles is transformed into kinetic energy
• This causes the particles to vibrate more and start to move / flow
• Melting happens at a specific temperature, known as the melting point (m.p.)

Boiling / evaporation

• Boiling and evaporation are both when a liquid changes into a gas

  • However, there is a key difference between boiling and evaporation

• In boiling, heat / thermal energy causes bubbles of gas to form inside the liquid, allowing for liquid particles to escape from the surface and within the liquid

• Boiling happens at a specific temperature, known as the boiling point (b.p.)

• Evaporation occurs over a range of temperatures

  • It can happen at temperatures below the boiling point of the liquid

• Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquid's surface at low temperatures

• The larger the surface area and the warmer the liquid surface, the more quickly a liquid can evaporate

Freezing

• Freezing is when a liquid changes into a solid

• This is the reverse of melting and occurs at the same temperature as melting

  • So, the melting point and freezing point of a pure substance are the same
  • For example, water freezes and melts at 0 ºC

• Freezing needs a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature

Condensation

• Condensation occurs when a gas changes into a liquid on cooling and takes place over a range of temperatures
• When a gas is cooled its particles lose energy and when they bump into each other they lack the energy to bounce away again, instead, they group together to form a liquid

Sublimation

• When a solid changes directly into a gas
• This happens to only a few solids, such as iodine or solid carbon dioxide
• The reverse reaction also happens and is called desublimation or deposition

Changing states of matter

State changes require a change in the energy of the particles

Exam Hint Questions may show state changes with a reversible arrow: ⇌, which means that the process can go forwards and backwards.